The present invention relates to an improved safety vent device provided in a battery module, that is constructed with a plurality of sealed rechargeable battery cells, for releasing pressure when the internal pressure of the rechargeable battery cells has reached a predetermined value.
Alkaline rechargeable batteries such as nickel metal hydride batteries are superior in energy density and have been used in recent years for the power source of electric vehicles. Such mobile power source is normally sealed tightly so as to be maintenance-free. Another feature of the rechargeable battery used in such mobile power source is that it is essential to ensure the safety and reliability of the battery because of the requirements for high capacities. There is accordingly a need for an improved safety vent structure for releasing pressure when the internal pressure of the batteries reaches a certain value, that operates accurately and reliably.
FIG. 5 shows a known safety vent structure applied in a conventional battery module comprising a plurality of sealed rechargeable battery cells. A valve case 41 is formed with a vent 42 in the center in the bottom face thereof, and closed tightly with a valve cap 43, in which a release aperture 44 is formed at the outer periphery on the upper edge thereof. A valve body 45 made of a resilient material such as rubber is arranged within the space enclosed by the valve case 41 and the valve cap 43 in a compressed state.
Japanese Laid-Open Patent Application No. 8-31398 proposes another vent structure, as shown in FIG. 6, wherein a valve case 51 is formed with a vent 52 in its bottom, and an annular sealing projection 53 surrounding the vent 52. The valve case 51 is closed by a valve cover 54 formed with a discharge port 55. A valve body 56 having a seal 57 in tight contact with the annular sealing projection 53, and a spring 58 inserted between the valve body 56 and the valve case 54 for pressing the valve body 56 towards the vent 52, are arranged within the space defined by the valve case 51 and the valve cover 54.
With the safety vent structure shown in FIG. 5, there are variations in the pressure at which the valve body 45 operates, because the valve body 45 makes surface contact with the bottom of the valve case 41 and thereby seals the vent 42 with a surface. It is difficult to control the deformation of the valve body 45 because the valve body 45 is an integral body made of a resilient material such as rubber that has both functions of producing a pressing force and of providing a seal. Therefore, the pressure at which the valve body 45 operates can hardly be set suitably. Moreover, under high temperatures, the rubber-made valve body 45 expands, whereupon the contact area between the valve body 45 and the valve case 41 or valve cap 43 increases. Then the aforementioned pressure at which the valve body 45 operates is accordingly raised.
While the safety vent structure shown in FIG. 6 resolves such problems as described above, it has different drawbacks. That is, since the valve body 56 is pressed against the vent 52 with the spring 58, it is difficult to cause the pressing force of the spring 58 to act accurately and uniformly on the entire periphery of the annular sealing projection 53, and the pressure at which the valve body 56 operates cannot be made constant. In order to provide countermeasures against this problem, a complex structure will be necessary, leading to increase in the number of components and costs. Moreover, there is the problem that the valve body 56 does not always stay concentric with the valve case 51 and part of it collides with the inner wall of the valve case 51, and friction thus generated between the valve body 56 and the valve case 51 may cause unstable operation of the safety vent structure.
In view of these problems of the prior art, it is an object of the present invention to provide an improved safety vent device for a battery module comprising a plurality of sealed rechargeable batteries, that can operate reliably and stably. Specifically, it is an object of the present invention to provide a safety vent device wherein there is less variance in the pressure at which it operates, the pressing force exerted to a valve body toward a vent for providing a seal around the vent can be made uniform, and the valve body can be restricted to be concentric with a valve case, so that the safety vent device can operate suitably even under high temperatures. Furthermore, the present invention aims at providing a safety vent device that can be constructed with a reduced number of components and with a simple configuration, and manufactured at low cost.
To achieve the above object, a sealed rechargeable battery according to the present invention is provided with a safety vent device, comprising:
a vent that communicates with the interior of the rechargeable battery;
a sealing protrusion formed around said vent; and
a valve body for closing the vent, including a sealing portion that is in tight contact with the sealing protrusion, a rigid holder for supporting said sealing portion, and a resilient portion for pressing said rigid holder towards said vent.
With this construction, deformation of the sealing portion can be restricted and the pressure at which the vent device operates can be made constant, because the sealing portion is supported by the rigid holder and pressed against the sealing protrusion, with which it makes line contact. Moreover, since the rigid holder is biased with the resilient portion, the pressing force can be applied on the entire surface of the sealing portion uniformly, thereby evening the sealing pressure around the entire circumference. Thus, with such a simple and inexpensive structure, the vent device can operate appropriately and stably.
The safety vent device according to the present invention is basically comprised of three components, that is, a valve case in which the vent and the sealing protrusion are formed, a valve body housed in the valve case, and a valve cover formed with apertures for closing the valve case. Thus the safety vent device of the present invention can be manufactured with a reduced number of components and with a simple construction at low cost.
The safety vent device of the present invention can be favorably applied in a sealed rechargeable battery, for releasing pressure when the pressure inside the battery has reached a predetermined value.
If the rigid holder is formed with a connection hole, and the sealing portion and the resilient portion are formed integral with each other, being passed through said connection hole in the rigid holder, the vent body can be constructed of a single component obtained through a single manufacturing process, whereby the structure of the vent device can be further simplified and the cost further reduced. In this case also, it is of course possible to use different rubber-like materials for the sealing portion and the resilient portion, respectively.
If the resilient portion has a height substantially equal to or more than its dimension in a radial direction, variations in the compression of the resilient member can be readily accommodated, whereby application of the pressing force can be further stabilized, and the variation in the pressure at which the vent device operates further reduced.
If, in a state wherein there is a gap between the outer circumferential surface of the rigid holder and the inner circumferential surface of the valve case, the valve body is positioned concentric with the valve case, with part of the outer circumferential surface of the rigid holder being engaged with the inner circumferential surface of the valve case, then the valve case and the valve body are prevented from colliding with each other, and the pressure at which the vent device operates can be made constant even at high temperatures, when the rubber-made valve body tends to expand.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.